Improved zig-zag antennas were disclosed in U.S. Pat. No. 2,759,183. As described in this patent, a zig-zag antenna consists of a long conductor bent into sinuous or zig-zag curve and supported by insulators in a plane parallel to a flat metal sheet at distances of the order of one-tenth of a wavelength at which the antenna is to be used. The effective electrical length of a complete convolution, as measured along the conductor, is made one wavelength long. The overall length of the antenna is usually between 6 and 10 wavelengths.
There are several ways of supplying power to zig-zag antennas, among them there are two methods for feeding the antenna at the half-way point of the long conductor. These two methods may be referred to for brevity as: (a) series feed and (b) parallel feed.
The series feed is symmetrical and, therefore it results in symmetrical patterns, but it requires a balun and a transformation from a rather high impedance of the order of 500 ohms looking into the two halves of the antenna conductors in series with each other down to the 75 or 50 ohm characteristic impedance of a coaxial transmission line which is likely to be used to feed the antenna. Such baluns, together with large ratio impedance transformations, are not only expensive but also, in many cases, impose a limit of their own on the maximum power handling capacity of the antenna.
The parallel feed is accomplished by making the two halves of the antenna come together and meet each other in cusp, usually at the center of the antenna. The cusp where the two conductors meet is then connected to the inner conductor of a coaxial transmission line. The outer conductor of this transmission line is connected to the metal sheet near which the zig-zag antenna is mounted. The parallel feed requires no balun and the impedance transformation ratio is decreased by a factor of 0.25.
If one visualizes an imaginary plane that is perpendicular to the supporting sheet and which passes through the long axis of the zig-zag antenna, it is found that the portion of the zig-zag in the neighborhood of the parallel feed point is asymmetrical. When the long axis of the antenna is vertical, the effect of the asymmetry is to distort the horizontal radiation pattern of the antenna. This distortion includes a change in the direction of the maximum radiation, a change in the phase distribution with azimuth angle and a change in the shape of the pattern. These effects are particularly undesirable when one is attempting to obtain an approximately circular radiation pattern in the horizontal plane by making use of four zig-zag antennas mounted on the four faces of a square vertical metal cylinder. What is obtained is not a circular pattern with a shallow ripple, but a pattern with deep ripples of non-sinusoidal shape. The ratios of the maxima to minima in the horizontal pattern is often too large to meet specifications of the desired circularity.
Similar undesirable effects are encountered when three zig-zag antennas are used on three faces of the cylinder to obtain an approximation to a cardiodial pattern or when two zig-zags are used to obtain a radiation pattern which is intended to result in even coverage of, say, 120.degree. of the horizon.